[gcc r12-2738] vect: Tweak dump messages for vector mode choice

[gcc r12-2738] vect: Tweak dump messages for vector mode choice

[Richard Sandiford]

https://gcc.gnu.org/g:315a1c3756cbc751c4af0ce0da2157a88d7c3b09

commit r12-2738-g315a1c3756cbc751c4af0ce0da2157a88d7c3b09
Author: Richard Sandiford <richard.sandiford@arm.com>
Date:   Wed Aug 4 16:52:08 2021 +0100

    vect: Tweak dump messages for vector mode choice
    
    After vect_analyze_loop has successfully analysed a loop for
    one base vector mode B1, it considers using following base vector
    modes to vectorise an epilogue.  However, for VECT_COMPARE_COSTS,
    a later mode B2 might turn out to be better than B1 was.  Initially
    this comparison will be between an epilogue loop (for B2) and a main
    loop (for B1).  However, in r11-6458 I'd added code to reanalyse the
    B2 epilogue loop as a main loop, partly for correctness and partly
    for better costing.
    
    This can lead to a situation in which we think that the B2 epilogue
    loop was better than the B1 main loop, but that the B2 main loop is
    not better than the B1 main loop.  There was no dump message to say
    that this had happened, which made it look like B2 had still won.
    
    gcc/
            * tree-vect-loop.c (vect_analyze_loop): Print a dump message
            when a reanalyzed loop fails to be cheaper than the current
            main loop.

Diff:
---
 gcc/tree-vect-loop.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/gcc/tree-vect-loop.c b/gcc/tree-vect-loop.c
index 00a57b2ba62..48a54b0957f 100644
--- a/gcc/tree-vect-loop.c
+++ b/gcc/tree-vect-loop.c
@@ -3064,7 +3064,16 @@ vect_analyze_loop (class loop *loop, vec_info_shared *shared)
 			= opt_loop_vec_info::success (main_loop_vinfo);
 		    }
 		  else
-		    delete main_loop_vinfo;
+		    {
+		      if (dump_enabled_p ())
+			dump_printf_loc (MSG_NOTE, vect_location,
+					 "***** No longer preferring vector"
+					 " mode %s after reanalyzing the loop"
+					 " as a main loop\n",
+					 GET_MODE_NAME
+					   (main_loop_vinfo->vector_mode));
+		      delete main_loop_vinfo;
+		    }
 		}
 	    }